Abstract

While looking at a package of dry yeast it is hard to believe that the package has organisms in it that are alive. But add the right ingredients and presto, the mixture becomes a bubbly, oozing, mess of life! What are the right ingredients? What does that yeast need to become active? Do this science project and figure it out for yourself!

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Introduction

Yeasts are tiny, microscopic organisms — or microorganisms — that are a type of fungus. This means that they are more closely related to a mushroom than to plants, animals, and bacteria (which are also microorganisms). You can see a picture of some yeasts in Figure 1, below. These little critters might sound strange and different, but people have actually been using them for thousands of years to make bread "rise." How does this work? It has to do with the metabolism of the yeasts, or, in other words, what they eat and what they turn that food into.

Figure 1. This image shows what individual yeasts look like using a powerful microscope. These are specifically baker's yeast (Saccharomyces cerevisiae).

Yeasts must get their food from their surrounding environment to grow and reproduce, or make more yeast. What do they eat? Yeasts feed on sugars and starches, which are in bread dough. They turn their food into energy and release carbon dioxide (CO2) gas as a result. This process is known as fermentation. The CO2 gas made during fermentation is what makes a slice of bread so soft and spongy. The pockets of gas are produced by yeasts when the dough is allowed to rise before baking. Fermentation is also used to make beer, wine, and champagne.

In this microbiology science project, you will grow yeasts in different conditions to see which cause the yeasts to be most active during fermentation. Some of the conditions you will test will change the pH of the environment that the yeasts will be grown in. pH is a scale that measures how acidic or how basic something is. A base has a pH that is above 7, such as baking soda or bleach, and an acid has a pH that is less than 7, such as lemon juice or battery acid. A neutral substance has a pH of 7, such as drinking water. For more on this topic, see the Science Buddies page on Acids, Bases, & the pH Scale. These are the five different conditions you will investigate in this science project:

Water only: This condition will be your negative control because it will not have the food (i.e., sugar) that yeasts need to eat and grow. The yeasts should remain inactive.

Sugar: This condition will be your positive control because it will have the sugar that yeasts need to eat and carry out fermentation.

Sugar and salt: The salt in this condition will increase the salinity (or amount of salt) that is in the yeasts' environment.

Sugar and baking soda: The baking soda in this condition will increase the environment's pH, making it more basic.

Sugar and vinegar: The vinegar in this condition will decrease the environment's pH, making it more acidic.

How do you think increasing the salinity will affect the ability of yeasts to carry out fermentation? What about making the environment more basic or more acidic? To measure how well the yeasts carry out fermentation, you will use balloons to trap the CO2 gas released by the yeasts during fermentation. To measure how much CO2 is in each balloon you will then submerge each under water and use water displacement to measure how much space each balloon takes up. Which condition(s) will cause the formation of the most CO2 gas and the most active fermentation?

Terms and Concepts

Yeasts

Microorganisms

Fungus

Carbon dioxide (CO2)

Fermentation

pH

Bases

Acids

Neutral substances

Water displacement

Questions

What do yeasts eat?

What do yeasts turn their food into?

How can the activity of yeast be measured?

What do you think will be the best conditions for yeast to ferment?

Bibliography

You can find out more about yeast and fermentation through these resources:

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Materials and Equipment

Plastic 2 L bottles (5). If you do not want to clean out and reuse the same bottles after each trial, you will need 10 additional bottles.

Permanent marker

Measuring spoons

White table sugar

Salt

Baking soda

Vinegar

Graduated, metric beaker, at least 100 mL in size. This may be purchased through a scientific supply company or online suppliers such as
Amazon.com
Alternatively, a clear liquid measuring cup may be used.

Warm tap water

A large pot or bowl, at least 5 qt. in size

Optional: Thermometer for measuring the water temperature. A suitable thermometer may be purchased locally or through online suppliers such as
Amazon.com

Dry yeast (at least 30 packets or an equivalent amount from a jar). Tip: Buying a whole jar is probably more economical than individual packets.

Latex balloons, around 9 to 12 inches inflated size (5). These may be found at discount and/or party-supply stores, or through online suppliers such as
Amazon.com.

Clock or timer

String or clips for tying off balloons

A large pan or bowl that the large pot or bowl can sit inside of

Optional: Adult helper

Lab notebook

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Experimental Procedure

Make a data table like Table 1, in your lab notebook for you to record your results in.

Water Displaced (mL)

Bottle

Condition

Trial 1

Trial 2

Trial 3

Average

#1

Water only

#2

Sugar

#3

Sugar and salt

#4

Sugar and baking soda

#5

Sugar and vinegar

Table 1. In your lab notebook, make a data table like this one to record your observations and results in. You will be recording how much water is displaced in milliliters (mL).

Prepare and label the water bottles for your experiment. Rinse each bottle thoroughly with water and remove any labels. Number each bottle 1–5 with the permanent marker.

Add 2 tablespoons (tbsp.) of sugar to bottles 2-5.

Do not add sugar to bottle #1 because it is the negative control.

Bottle #2 will only have sugar and will be a positive control.

Add 2 tbsp. of salt to bottle #3. How do you think increasing the environment's salinity will affect yeast fermentation?

Add 2 tbsp. of baking soda to bottle #4. How do you think being in a basic environment will affect yeast fermentation?

Add 2 tbsp. of vinegar to bottle #5. How do you think being in an acidic environment will affect yeast fermentation?

Fill a large pot with at least 4 liters (L) (or 17 cups [C]) of very warm tap water.

Adjust the temperature of the hot water coming from the tap until it is almost too hot to hold your hands under. Use this temperature water to fill the pot.

If you have a thermometer, you can measure the temperature of the water and confirm that it matches the recommended temperature on the yeasts' packaging. If you do this, record this temperature in your lab notebook.

Using the water from the pot, to each bottle add 600 milliliters (mL) (about 2.5 C, or 1/3 full) of warm water, replace the lid, and shake thoroughly to dissolve all of the ingredients.

Using the water from the pot will help you make sure that the water in each bottle is the same temperature.

To each bottle add two packets of dry yeast (or an equivalent amount from a jar), replace the bottle's lid, and shake gently to mix in the yeast.

Remove each lid and stretch a balloon completely over the opening of the bottle, as shown in Figure 2.

Figure 2. Stretch the balloon completely over the bottle's opening, going over all of the bottle's ridges, as shown here.

Leave the bottles to rest in a warm location for 1 hour.

Note: Be sure the balloons stay out of direct sunlight as this can cause the balloons to deflate.

After 1 hour, look at the balloons. Which ones have become inflated? How big are they compared to each other? Write your observations in your lab notebook.

Using a permanent marker, number each balloon so it matches its bottle.

Carefully tie a length of string tightly around the base of each balloon to seal the balloon so that any gas inside cannot escape. When the balloon is tightly sealed, remove it from the bottle top, being very careful not to loosen the string or allow any air to escape from the balloon.

After removing each balloon from its bottle, carefully tie a knot at the bottom of each balloon (where the string is) so that you can be sure no air will escape the balloons while you collect your data.

Now you are ready to measure the amount of CO2 gas inside each balloon with a water displacement test. To do this, do the following:

Fill the large pot or bowl completely full with water, to the very top of the pot.

Carefully place the pot in the pan or bowl that it can fit inside of, as shown in Figure 3. You may need an adult helper to help you move the pot. If you spill any water while doing this, clean up the spill and add water back to the large pot so it is completely full again.

Figure A. This is an example of the wind tunnel you can make, based on the design and instructions provided in this how-to guide!

Take one of the balloons and slowly dunk it into the large pot of water. Water should spill out and go into the pan that the large pot is sitting in. Dunk the balloon until it is completely submerged.

Make sure not to dunk your hand in.

If the pan becomes full of water before the balloon is completely submerged, go on to step 16.d. and then repeat steps 16.b. to 16.d. until you can completely submerge the balloon.

Once you have submerged the balloon, carefully lift the pot off of the pan. Pour the water from the pan into the graduated, metric beaker.

In the data table in your lab notebook, record how much water was displaced (how much water is in the beaker) by the balloon, in milliliters. This should be recorded in the column for Trial 1. Then empty the measuring cup.

Repeat steps 16.a. to 16.e. until you have done the water displacement test for each of the five balloons.

If you only have 5 bottles, rinse them out thoroughly so you can reuse them in the next trial. If you have 15 bottles, repeat step 2 with 5 unused ones so they are ready to use.

Repeat steps 3–17 two more times so that you have done a total of three trials. Each time be sure to record your results in your data table for the correct trial number.

Calculate the average water displaced for each condition over the three trials. Record the results in your data table.

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Variations

Another environmental condition that can affect the activity of yeast and the process of fermentation is temperature. Prepare several bottles using the same conditions. Now place each bottle in a different place with a different temperature. After one hour, measure the amount of gas produced and compare.

In this experiment, you added the same amount of sugar to each bottle because it is what the yeast uses to make CO2 gas during fermentation. If you add more sugar, will you always get more CO2 gas? How much more sugar can you add? Is there a limit?

You could try this science project again but instead of comparing salt, baking soda, and vinegar, you could try comparing different types of sugars or sugars from different sources, such as from various juices.

More advanced students may want to try these Science Buddies projects using yeast:

What was the most important thing you learned?
Yeast like sugar, and don't like vinegar.

What problems did you encounter?
Need more specific information on some of the steps. For example, the balloons we got were small baloons, but did not fit the lip of the bottle, so a note suggesting that in the materials required section would be useful. Also, in step 2, it should read add sugar to bottles 2-5 instead of 2-4.

Can you suggest any improvements or ideas?
Possible additional conditions to test would be to start with different types of water (tap/filtered/bottled/river/sea etc.), different types of sugar (white/brown/raw/powdered/glucose/corn syrup etc.)

Overall, how would you rate the quality of this project?
Very Good

What is your enthusiasm for science after doing your project?

Compared to a typical science class, please tell us how much you learned doing this project.

What was the most important thing you learned?
I learned that yeast ferments the best with just the right conditions. It needs sugar, doesn't like lots of salt, but does like acid.

What problems did you encounter?
Water balloons definitely don't work! Party balloons worked great, but you might add a note about size of the balloons needed. "Small" suggests water balloon size could work.

Can you suggest any improvements or ideas?
See above.

Overall, how would you rate the quality of this project?
Very Good

What is your enthusiasm for science after doing your project?

Compared to a typical science class, please tell us how much you learned doing this project.

Ask an Expert

The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Related Links

If you like this project, you might enjoy exploring these related careers:

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Read more

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